The present invention relates to a wavelength measuring apparatus for measuring wavelength characteristics of a laser beam oscillating from a vacuum ultraviolet laser.
Conventionally there is known a vacuum ultraviolet laser emitting a laser beam 11 having a wavelength of approx. 20 nm to 200 nm referred to as vacuum ultraviolet region such as, for example, ArF lasers (193 nm) and F2 lasers (157 nm).
This type of vacuum ultraviolet laser is mainly used for precision processing such as laser lithography or the like. To favorably perform the precision processing, it is necessary to mount a wavelength selecting element on the vacuum ultraviolet laser so as to stabilize a center wavelength of the laser beam applied to an object to be processed and to narrow a spectral width of the wavelength (it is referred to as narrowing a laser beam band).
Furthermore, to preferably perform the precision processing, the center wavelength and the spectral width (hereinafter, generally referred to as wavelength characteristics) of the above laser beam need to be limited within a predetermined allowable range. For this purpose, the wavelength characteristics of the laser beam 11 having the narrowed band must accurately be measured and controlled on the basis of the measured values.
Referring to FIG. 5, there is shown a configurational view of an F2 laser unit having a wavelength measuring apparatus related to a prior art.
In FIG. 5, the F2 laser unit 1 comprises a laser chamber 2 enclosing laser gas and emitting a laser beam 11 by causing an electric discharge inside, a band narrowing unit 10 for narrowing a band of the laser beam 11 emitted from the laser chamber 2, a wavelength measuring apparatus 3 for measuring wavelength characteristics of the laser beam 11, and a wavelength controller 4 for controlling the wavelength characteristics of the laser beam 11 whose band has been narrowed so as to be limited within an allowable range with being electrically connected to the wavelength measuring apparatus 3 and the band narrowing unit 10.
The laser chamber 2 encloses laser gas such as, for example, fluorine (F2) and helium (He) at a predetermined pressure ratio. A pair of discharge electrodes (not shown) are installed in a predetermined position inside the laser chamber 2 and the laser beam 11 is caused to oscillate by applying a high voltage between the discharge electrodes.
A rear window 9 at a rear end (left-handed in the drawing) of the laser chamber 2 transmits the oscillating laser beam 11 and then the laser beam is incident on the band narrowing unit 10 arranged externally at the back of the laser chamber 2. Inside the band narrowing unit 10, an etalon, a grating, or other wavelength selecting elements (not shown) are arranged in predetermined positions to narrow the band of the laser beam 11.
The laser beam 11 whose band has been narrowed passes through the laser chamber 2 and penetrates through a front window 7 at a front end of the laser chamber 2, and then a part of it partially penetrates through a front mirror 8 arranged externally ahead of the laser chamber 2 to be emitted to the outside.
At this point, a beam splitter 12 is arranged on an optical axis of the laser beam 11 in order to measure the wavelength characteristics of the emitted laser beam 11. The laser beam 11 is partially reflected downward by the beam splitter 12 to generate a sample beam 11A and it is incident on the wavelength measuring apparatus 3, so that its wavelength characteristics are measured.
The wavelength measuring apparatus 3 comprises a diffuser panel 24 for diffusing the sample beam 11A, a monitor etalon 25 for generating an interference pattern 29 corresponding to the wavelength characteristics of the diffused sample beam 11A, a first imaging lens 27 for imaging this interference pattern 29, a pattern detector 17 (for example, a line sensor) for measuring an intensity distribution of the imaged interference pattern 29, and an arithmetic unit 28 for calculating the wavelength characteristics of the sample beam 11A on the basis of an output from the pattern detector 17.
This arithmetic unit 28 transmits the wavelength characteristics of the calculated sample beam 11A to the wavelength controller 4. The wavelength controller 4 outputs a command signal to the band narrowing unit 10 on the basis of the wavelength characteristics and controls the band narrowing unit 10 so that the wavelength characteristics of the laser beam 11 are limited within a predetermined range. This feedback control enables the wavelength characteristics of the laser beam 11 to be controlled.
The prior art set forth in the above, however, has problems described below.
In other words, the interference pattern 29 has the same wavelength as for the laser beam 11. Light in the vacuum ultraviolet region has a short wavelength and receives very large energy of photons inversely proportional to a wavelength. Therefore, the pattern detector 17 is damaged by the energy of photons of the incident interference pattern 29, by which the wavelength characteristics cannot be measured accurately.
In addition, this causes the measured values of the wavelength characteristics of the laser beam 11 to be inaccurate, which results in a fluctuation of the wavelength characteristics of the laser beam 11 controlled by the wavelength controller 4 on the basis of the measured values. It further fluctuates the wavelength characteristics of the laser beam 11 applied to an object to be processed, which results in a fluctuation of a focal position of the laser beam 11 inside a processing machine which is not shown and causes a precision processing failure.
Furthermore, the wavelength characteristics are measured at all times during processing and a signal for halting the processing is outputted to the processing machine when the wavelength characteristics deviate from a predetermined range. In this condition, the inaccurate measurements of the wavelength characteristics causes the processing to be halted in spite of favorable wavelength characteristics or to be continued in spite of poor wavelength characteristics.
The present invention has been provided in view of the above problems. It is an object of the present invention to provide an apparatus for measuring a wavelength of a vacuum ultraviolet laser, capable of accurately measuring wavelength characteristics of the laser beam.
To achieve the above object, in accordance with a first aspect of the present invention, there is provided a vacuum ultraviolet laser wavelength measuring apparatus having spectral means for generating an optical pattern corresponding to wavelength characteristics of an incident laser beam and measuring wavelength characteristics of a laser beam in a vacuum ultraviolet region oscillating from a vacuum ultraviolet laser on the basis of the optical pattern, comprising: a fluorescent screen for generating a fluorescent pattern having an intensity distribution corresponding to an intensity distribution of the incident optical pattern, a pattern detector for measuring the intensity distribution of the fluorescent pattern generated from the fluorescent screen, and an arithmetic unit for calculating the wavelength characteristics of the laser beam on the basis of the intensity distribution of the measured fluorescent pattern.
With these features, an interference pattern or other optical pattern generated by the spectral means is caused to be incident on the fluorescent screen, the intensity distribution of the fluorescent pattern generated from the fluorescent screen is measured by the pattern detector, and the wavelength characteristics of the laser beam are calculated by the arithmetic unit on the basis of the measured values.
As set forth in the above, the wavelength characteristics of the laser beam can be measured without causing the laser beam to be directly incident on the pattern detector by measuring the intensity distribution of the fluorescent pattern. A fluorescent light has a longer wavelength than the laser beam in the vacuum ultraviolet region and has a small energy of photon, and therefore it does not damage the pattern detector unlike the laser beam directly incident. This reduces troubles of the wavelength measuring apparatus, thereby improving an operating efficiency of the vacuum ultraviolet laser.
In addition, the wavelength characteristics of the laser beam from the vacuum ultraviolet laser can always be measured accurately, thereby enabling a precise control of the wavelength characteristics based on the measured values. This makes it possible to irradiate an object to be processed with a laser beam having wavelength characteristics within a predetermined allowable range when the vacuum ultraviolet laser is used as a light source for precision processing such as laser lithography.
Preferably according to a second arrangement, the fluorescent screen in the first aspect of the invention is coated with a fluorescent substance on its surface in one side, the optical pattern is caused to be obliquely incident on the fluorescent screen at a predetermined incident angle, a fluorescent pattern having an intensity distribution corresponding to an intensity distribution of the obliquely incident optical pattern is generated on the surface in the side, and the fluorescent pattern is imaged on the pattern detector by using a second imaging lens.
According to the second arrangement, the fluorescent screen is coated with a fluorescent substance on its surface in the side, the optical pattern is caused to be obliquely incident on the fluorescent screen, the fluorescent pattern having the intensity distribution corresponding to the intensity distribution of the incident optical pattern is generated on the surface in the side, and the fluorescent pattern is imaged on the pattern detector, by which the same action and effect as for the first aspect of the invention is achieved.
A third arrangement may be such that the spectral means in the first aspect of the present invention comprises a plurality of concave mirrors and a diffraction grating for diffracting a laser beam at an angle corresponding to the wavelength characteristics of the incident laser beam, wherein the fluorescent pattern having an intensity distribution corresponding to an intensity distribution of the diffracted beam is generated in the other side of fluorescent screen and wherein the fluorescent pattern is imaged on the pattern detector by using the second imaging lens.
According to the third arrangement, the spectral means comprises the plurality of concave mirrors and the diffraction grating for diffracting the laser beam at the angle corresponding to the wavelength characteristics of the incident laser beam, wherein the fluorescent pattern having the intensity distribution corresponding to the intensity distribution of the diffracted beam is generated on the fluorescent screen and wherein the fluorescent pattern is imaged on the pattern detector, by which the same action and effect as for the first aspect of the invention is achieved.
A fourth arrangement according to the first, second, or third arrangement may further comprise an ultraviolet filter which does not transmit the laser beam in the vacuum ultraviolet region and transmits the laser beam having a wavelength close to a wavelength of the fluorescent pattern in the front of the pattern detector.
According to the fourth arrangement, there is provided an ultraviolet filter which does not transmit the laser beam in the vacuum ultraviolet region in the front of the pattern detector and therefore the laser beam irregularly reflected on a surface or the like of the fluorescent screen does not reach the pattern detector, thereby preventing the pattern detector from being damaged.
A fifth arrangement according to the first, second, or third arrangement may be such that a cover is put on at least an entire optical path of the laser beam and the wavelength measuring apparatus and a space inside the cover is kept in an oxygen-free condition.
According to the fifth arrangement, the cover is put on at least the entire optical path of the laser beam and the wavelength measuring apparatus and the space inside the cover is kept in the oxygen-free condition, by which the laser beam is not absorbed by oxygen in the air, thereby preventing its power from attenuating.
A sixth arrangement according to the first, second, or third arrangement may be such that the fluorescent screen has a movable actuator which is movable by a predetermined distance perpendicularly to an optical axis of the incident laser beam and is movably installed by this movable actuator.
According to the sixth arrangement, the fluorescent screen can be moved by the predetermined distance by the movable actuator, thereby reducing deterioration of the fluorescent substance and therefore extending a life of the fluorescent screen.